(a) Fields of the Invention
The present invention relates to methods for forming a ferroelectric film made of insulating metal oxide and to semiconductor devices including such ferroelectric films.
(b) Description of Related Art
With recent progress in digital technologies, there is a growing trend to rapidly process or store a large capacity of data. In accordance with this trend, higher packing density and higher performance of semiconductor devices used in electronic equipment are demanded. To achieve higher packing density and lower power consumption of the semiconductor devices, research and development is widely conducted on the technology to employ, for capacitor elements in the devices, capacitor insulating films made of films of high dielectric constant instead of conventional capacitor insulating films made of silicon oxide or silicon nitride. Moreover, with the aim of putting into practical use nonvolatile RAMs capable of performing rapid writing and reading operations at conventionally impracticable low operating voltage, research and development is actively conducted on ferroelectric films as capacitor insulating films having spontaneous polarization properties.
Of semiconductor memory devices employing high dielectric constant films or ferroelectric films as capacitor insulating films, those with ultrahigh packing density use three-dimensional memory cells as alternatives to conventional stacked memory cells.
In fabricating a three-dimensional memory cell, a capacitor insulating film made of a ferroelectric film has to be formed on a lower electrode with steps. Therefore, practical utilization of capacitor insulating film formation methods using CVD techniques capable of providing excellent step coverage is strongly demanded. In particular, one method for forming a capacitor insulating film of a ferroelectric film using metal organic chemical vapor deposition (referred hereinafter to as MOCVD) is a method for forming bismuth oxide by MOCVD using bismuth organometallic compound with bismuth-oxygen bonds as a material (see, for example, Japanese Unexamined Patent Publication No. 09-142844, (page 5, paragraph 27)).
A conventional method for forming a ferroelectric film will be described below with reference to FIG. 19.
Referring to FIG. 19, a first material container 101 made of stainless steel is provided within a first constant temperature bath 100. Bismuth tributoxide filled in the first material container 101 is heated to 80 to 110° C. Argon gas is introduced into the first material container 101 at a flow rate of 50 to 100 ml/min (in a normal state), and then bismuth tributoxide is sublimated at an elevated temperature and a reduced pressure. Subsequently, the sublimated bismuth tributoxide is introduced into a gas line 102 kept at about 110° C. and carried to an MOCVD reaction chamber 103.
A second material container 105 is provided within a second constant temperature bath 104. Tantalum pentaethoxide (Ta(OC2H5)5) filled in the second material container 105 is heated to 120° C. and then bubbled using argon gas with a flow rate of 50 to 100 ml/min (in a normal state). Subsequently, the gasified tantalum pentaethoxide (Ta(OC2H5)5) is introduced into a gas line 106 of stainless steel heated to 130° C. and carried to the MOCVD reaction chamber 103. Strontium dipivaloylmethane tetraethylenepentamine (C38H84O4N10Sr) filled in a third material container (not shown) is heated to 150° C. and then bubbled using argon gas with a flow rate of 50 to 100 ml/min (in a normal state). The gasified strontium DPM tetraethylenepentamine is introduced into a gas line (not shown) of stainless steel heated to 160° C. and carried to the MOCVD reaction chamber 103.
The MOCVD reaction chamber 103 is equipped with a base stage 108 which has a base 107 of platinum (Pt) provided on its upper surface. Onto the surface of the base 107 kept at 400 to 800° C., preferably at 450 to 700° C., the three types of source gases shown above, that is, bismuth tributoxide, tantalum pentaethoxide (Ta(OC2H5)5), and strontium dipivaloylmethane tetraethylenepentamine (C38H84O4N10Sr) are simultaneously introduced accompanied with oxygen gas and argon gas for dilution. Then, a ferroelectric film made of insulating oxide containing Bi, Sr, and Ta is formed on the surface of the base 107.
In order to obtain a ferroelectric film containing components in desired proportions (Bi:Sr:Ta=2:1:2), it is sufficient that the flow rates of argon gases introduced into the first to third material containers or the heating temperatures of the first to third material containers are adjusted to control supplies of the source gases to the MOCVD reaction chamber 103. This control can grow a ferroelectric film made of insulating oxide represented by SrBi2Ta2O9.
We conducted various studies on the conventional example mentioned above. From these studies, we found the following fact: in forming a ferroelectric capacitor element which has a capacitor insulating film made of a ferroelectric film, it is difficult, by simply adjusting the flow rates of the argon gases or the heating temperatures of the source gases, to realize the ferroelectric film having an excellent step coverage over an electrode of the element and concurrently to control the components of the ferroelectric film to desired proportions.
More specifically, to provide the ferroelectric film of an SBT film composed in proportions of Sr:Bi:Ta:O=1:2:2:9, we adjusted not only the flow rates of the argon gases or the heating temperatures of the source gases but also the mixture ratio of the source gases introduced in the MOCVD reaction chamber 103. Then, we were able to form a ferroelectric film composed in a desired composition represented by SrBi2Ta2O9.
In this case, in order to provide the ferroelectric film composed in proportions of Sr:Bi:Ta:O=1:2:2:9, the MOCVD reaction chamber 103 was supplied with sufficient amounts of the source gases. Therefore, the chemical reaction within the MOCVD reaction chamber 103 proceeded depending on the rate of the gas supplies, which caused the disadvantage that the ferroelectric film having an excellent step coverage over the electrode cannot be formed. In particular, for a capacitor insulating film used for a three-dimensional ferroelectric capacitor element formed with an ultrafine pattern necessary for high integration, we found the disadvantage that desired polarization properties cannot be obtained because of an inadequate step coverage of the ferroelectric film over the electrode.
On the other hand, a ferroelectric film made of an SBT (SrBiTa) film was formed by adjusting the flow rates of the argon gases or the heating temperatures of the source gases to develop the chemical reaction within the MOCVD reaction chamber 103 depending on the reaction rate of the gases. Then, we were able to form the ferroelectric film having an excellent step coverage over the electrode. From this result, in order to form a ferroelectric film of an SBT film containing components in desired proportions with the chemical reaction for film growth proceeding depending on the reaction rate of the gases, a fine adjustment of the flow rates of the argon gases or the heating temperatures of the source gases was made to form a ferroelectric film. In this approach, however, the proportions of components of the formed ferroelectric film made of an SBT film could hardly be changed, and therefore the formed film failed to have desired polarization properties.